Radio frequency (RF) filters have been used with cellular base stations and other telecommunications equipment for some time. Such filters are conventionally used in a receiver front-end to filter out noise and other unwanted signals that would harm components of the receiver in the base station. For example, bandpass filters are conventionally used to filter out or block RF signals in all but one or more predefined bands. With the recent dramatic rise in wireless communications, such filtering should provide high degrees of both selectivity (the ability to distinguish between signals separated by small frequency differences) and sensitivity (the ability to receive weak signals) in an increasingly hostile frequency spectrum.
The relatively recent advancements in superconducting technology have given rise to a new type of RF filter, namely, the high-temperature superconducting (HTS) filter. HTS filters contain components that are superconductors at or above the liquid nitrogen temperature of 77K. Such filters provide greatly enhanced performance in terms of both sensitivity and selectivity as compared to a conventional filter. HTS components have been utilized in bandpass filters disposed in the receive path of a cellular base station.
Many front-end systems in the cellular and PCS (personal communication systems) industries utilize the same antenna for both reception and transmission. As a result, certain base stations have required a duplexed front-end. In the past, such duplexed configurations have included a stand-alone duplexer coupling the antenna to an RF filter in the receive path.
Base station installations have also required low losses as well as high selectivity. In some cases, an HTS bandpass filter has been incorporated into the receive path. When disposed in a duplexed configuration, however, losses were still undesirably introduced via the components in the duplexer. In addition, having both a stand-alone duplexer and an HTS filter added size as well as complexity to the system.
Generally speaking, duplexed receive configurations must address the significantly different power levels experienced by the receive in a simplex configuration. Utilization of an HTS bandpass filter has therefore complicated the configuration of the duplexed front-end, inasmuch as the use of such a filter in the transmit path would require significantly more cooling capacity. Prior duplexed front-ends have accordingly combined an HTS filter in the receive path with a conventional duplexer. However, the overall system suffered additional losses due to the additional connection between the HTS filter and the duplexer.
Typically, a duplexer constitutes a device or set of devices that utilizes the delay between the transmission of a pulse and the echo thereof to permit the connection of both a transmitter and a receiver (i.e., a transceiver) to a common antenna. Duplexers (or duplexed configurations) have been realized in a number of ways, including through combinations of components, such as 3-dB hybrids and bandpass filters, as well as, more generally, via the adjustment of the phase of the incoming and outgoing signals to prevent the undesired propagation of transmit signals in the receive path, and vice versa. For example, the phase of an incoming (i.e., received) signal may be adjusted such that it will not be propagated down the transmit path of the transceiver system by adjusting the length of the transmission line between the antenna and the components in the receive and transmit paths. One approach in adjusting the transmission line length is by installing additional cabling of a certain length between the antenna and an RF filter in the receive path.
However, adding such cabling undesirably adds to the losses encountered in the receive path. The noise figure for the receive path of the base station is, in large part, set by the losses introduced upstream of any amplification of the desired signal. As a result, any advantage gained through the use of low-loss components, such as an HTS bandpass filter and low-noise amplifier (or LNA), would be lost.
In prior base station installations, the receiver front-end has often been disposed on the tower supporting the antenna to minimize the length of such cabling, thereby minimizing any pre-LNA losses. Regardless of whether the front-end is disposed in a tower-top installation, it has generally been desirable to minimize cable lengths prior to the HTS filter and LNA in order to realize as much advantage from the HTS filter as possible.